[PaulStatezny]

To answer your question, let's consider the ways in which the videos do _not_ work together:

- They're not playing in the same tempo

- They're not playing the same note lengths

- They're not necessarily playing notes in the same chords

It's somewhat analogous to sitting down at a piano, holding down the sustain pedal (so that all the notes played ring out for a long time), and just playing notes in the same key. (Say, all the white keys.)

It "works" because the notes are in the same key. But note that we're not making music with any kind of groove or consistent beat. It's more like an ethereal, improvised holding out of a single chord.

TL;DR – "Or is the whole point that they didn't agree to anything upfront and it still somehow works?" Yes :)

[1000100_1000101]

Even shorter TL;DR: It's Youtube wind chimes.

[Ylpertnodi]

Just use the black keys.

[PaulStatezny]

Pentatonic scale :)